[PATCH] D90125: [CSSPGO] Infrastructure for context-sensitive Sample PGO and Inlining

Tue Nov 3 09:50:51 PST 2020

wmi added a comment.

In D90125#2368991 <https://reviews.llvm.org/D90125#2368991>, @wenlei wrote:

> In D90125#2365948 <https://reviews.llvm.org/D90125#2365948>, @wmi wrote:
>
>> Another question. Have you ever evaluated the performance by comparing this patch "CSSPGO profile working with existing AFDO pipeline" with the default SPGO? I understand CSSPGO's benefit has not been used without the change of CGSCC inliner. The intention of the comparison is to understand how well existing AFDO pipeline work with CSSPGO profile. It may expose problem in existing SPGO profile or CSSPGO.
>
> Yeah, I've tried that initially on SPEC. It showed some perf win, however the problem with inliner (actually the sample loader inliner, not the CGSCC inliner) is very visible on a few cases. It's mostly because today's sample loader inliner is a replay inliner, so it won't be more aggressive than CGSCC inline from previous build, hence simple hotness heuristic works. But with CSSPGO profile, it's unbounded on hot path (as long as it's hot, there's no check on inlinee's size or inline cost) and can lead to size bloat and perf regression in some cases.

I see. Thanks. CSSPGO profile is currently oblivious to inline/outline, unlike current SPGO profile which has the concept of inline/outline instance. So CSSPGO profile cannot replace SPGO to drive the current early inliner (oblivious to inline size, used mainly for maximize profile matching).

> I have an upcoming change to make sample loader inliner a priority based inliner with size and cost checks. And it should work for today's AFDO as well - if we treat all call sites to inline with equal priority, and set inline limit to infinite, it should be a no-op change for AFDO, but perhaps can be tuned it to benefit AFDO later too.

If CSSPGO has the priority based inliner, does it mean it will do most inliner work and CGSCC inliner will mostly be used as an iterative clean up pass?

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